1. Field of the Invention
The present invention relates to a head position control system for a disc storage unit such as a fixed disc unit for displacing an information read/write head by a head control motor, especially such as a stepping motor, to a desired position and thereafter positioning the write/read head at a normal position and maintaining the head at this normal position.
2. Description of the Prior Art
Recently, there has been remarkable progress in the development of disc storage units, so that even in a fixed disc unit which uses a disc of 3.5 inches in diameter it is usual that the disc unit has a storage capacity of several tens megabits. In spite of the remarkable progress in storage capacity, the access time from a computer to desired information stored in a disc storage unit is quite long, compared with the access time to a RAM or ROM within the computer. In fact, the displacement of the write/read head for writing or reading data into or from a disc must be carried out mechanically, so that its control or access time is naturally limited, unlike electronic operations within the computer. In practice, the actual mechanical access time is considerably longer than a minimum access time required for carrying out a complete mechanical operation, so that there exists still room for shortening the access time.
In this case, there exists a contradictory problem when the access time is shortened while the data storage density is increased. More particularly, when a large number of tracks are defined on one major surface of a disc, even a very small error of the head position in the radial direction of the disc results in erroneous writing or reading of data, so that the head must be correctly controlled to be positioned at a normal position of any desired track. Therefore, the position of the head is controlled by a closed loop system in such a way that so-called servo information which is recorded on one major surface of a disc is read out by the head. In response to the servo information thus read out by the head, an offtrack amount of the head from its normal position is detected and the position of the head is so controlled that the off-track amount becomes zero.
In order to bring the head to a selected track at which desired information is written or read out, the head must be displaced from a certain track to the selected track. In this case, an open loop control system is used to control the position of the write/read head.
In practice, after the open loop control has been accomplished, the above-mentioned closed loop control is carried out. In any case, two different control systems are used, so that the control characteristics are different from each other and consequently a contradictory and unreasonable position control tends to occur. As a result, it may happen that the position control cannot be carried out as originally intended, so that in many cases an erroneous position control occurs, and consequently an unexpectedly long time period is required to correct such erroneous position control.
The above and other problems encountered in the conventional head position control system will be described in more detail with reference to FIGS. 6A-6E.
In the head position control, the direct object to be controlled is a head control motor which is mechanically coupled to the head. In general, a two-phase motor or more specifically a so-called stepping motor is used as the head position control motor. FIG. 6A shows waveforms of two phase currents Ia and Ib in case of the open loop control. The waveforms are so illustrated to correspond to the positions of composite phase current vectors in an electrical angle diagram shown in FIG. 6B. In FIG. 6B, eight reference vector positions are represented by numerals 0-7, respectively, which are also shown in the upper portion of FIG. 6A.
As is readily understood by those skilled in the art, ideal waveforms of the phase currents Ia and Ib change as illustrated by the broken lines in FIG. 6A, but in practice, because the phase coils of the head control motor have reactance, the actual phase currents Ia and Ib have waveforms illustrated by the solid lines in FIG. 6A which tend to lag behind the ideal waveforms. In this case, while the practical waveforms are quite complicated, relatively simplified waveforms are shown in FIG. 6A for the sake of simplicity. It is seen from FIG. 6A that the waveforms of the phase current Ia at, for instance, the vector positions 0 and 4 are considerably different from the ideal waveforms.
In the case of the open loop control described above, the velocity v of the write/read head or the velocity of the control motor is not maintained constant, but is varied in the form of a trapezoid or a triangle with respect to time t as shown in FIG. 6C or 6D, so that the head can be displaced by a predetermined distance in the shortest time period. It is apparent that FIG. 6C shows the case in which the displacement of the head is greater, while FIG. 6D shows the case in which the displacement of the head is shorter. The waveforms as shown in FIG. 6A are obtained on the upper flat side of the trapezoid shown in FIG. 6C. When the velocity v rises or falls, the time interval corresponding to a certain vector position becomes shorter than the time interval shown in FIG. 6A, so that the rise or fall of the velocity v is further influenced by reactance. In this case, if the repetition period of the phase current pulses as shown in FIG. 6A is shortened, in order to increase the displacement velocity of the head control motor, the driving torque is decreased and in the worst case, the head control motor would not respond to the given phase current pulses.
The position at which the head stops after the completion of the open loop control period is not necessarily the normal position of a selected track, and it frequently happens that the head stops at a position with a so-called off-track amount. Therefore, as shown by the broken-line arrow in FIG. 6B, it is required that the vector position applied to the head control motor be slightly shifted from the above-mentioned typical reference vector position, for instance, the vector position 0. To this end, the correction of the head position is carried out by closed loop control. In this case, a phase current having a current value which is more accurately determined than the phase current command value for the reference vector position in case of open loop control must be applied to the head control motor. The phase current is modulated, for instance, by pulse width moduration (PWM) as shown in FIG. 6E. More particularly, a predetermined time period is divided into, for instance, 16 periods as shown in such a way that the "ON" time becomes an integral multiple of a divided time period to correctly designate the phase current I. However, such a PWM phase current essentially has a repetitive pulse waveform, which is not necessarily adapted to be applied to the head control motor. However, in this case, the reactance of the phase coil serves to smooth the PWM phase current having the repetitive pulse waveform. Therefore, as is apparent from the above description, it is necessary that the reactance of each phase coil of the head control motor has a small value in the case of open loop control, whereas it is preferable that the reactance is rather large in the case of closed loop control.
After the head has been corrected to be brought to its normal position, the phase current having a correct current value as described above must be applied to the head control motor, so that data is written or read out while the head is maintained at the normal position. In this case, the head control motor functions not as a drive motor but as a kind of torque motor. During writing or reading data into or from a disc, it is important to apply a sufficient holding torque to the head control motor so that the head is prevented from being deviated from its normal position due to external causes such as vibrations. Of course, the holding torque of the head control motor is determined by the product of the phase current and the number of turns on the phase coil, but there arises a difficulty that if the reactance of each phase coil is decreased so as to improve the response time in the open loop control, the number of turns of each phase coil must be decreased so that the holding torque becomes insufficient to maintain the head at its normal position. Of course, it may be considered to increase the magnitude of the phase current while maintaining the head at its normal position. This method, however, naturally will increase the current consumption and hence the power consumption of the motor. In addition, the head control motor will be overloaded, so that this method is not satisfactory in practice.